During the early to middle Eocene, a mid-to-high latitudinal position and enhanced hydrological cycle in Australia would have contributed to a wetter and "greener" Australian continent where today arid to semi-arid climates dominate. Here, we revisit 12 Australian plant megafossil sites from the early to middle Eocene to generate temperature, precipitation and seasonality paleoclimate estimates, as well as net primary productivity (NPP) and vegetation type, based on paleobotanical proxies and compare to early Eocene global climate models. Temperature reconstructions are uniformly subtropical (mean annual, summer, and winter mean temperatures 19–21 °C, 25–27 °C and 14–16 °C, respectively). This indicates that southern Australia was ~5 °C warmer than today, despite a >20° poleward shift from its modern geographic location. Precipitation was less homogeneous than temperature, with mean annual precipitation of ~60 cm over inland sites and >100 cm over coastal sites. Precipitation may have been seasonal with the driest month receiving between 2–7× less precipitation as mean monthly precipitation. Proxy-model comparison is favorable with an 1680 ppm CO2 concentration. However, individual proxy reconstructions can disagree with models as well as with each other. In particular, seasonality reconstructions have systemic offsets. NPP estimates were up to 1000 gC m-2 yr-1 higher than modern, implying a more homogenously "green" Australian continent in the early to middle Eocene and larger carbon fluxes to and from the Australian biosphere. The most similar modern vegetation type is modern-day eastern Australian subtropical forest, although distance from coast and latitude may have led to vegetation heterogeneity.

Funding provided by: Natural Sciences and Engineering Research Council of Canada Discovery Grant*Crossref Funder Registry ID: Award Number: DG 311934Funding provided by: Natural Sciences and Engineering Research Council of Canada Discovery Grant*Crossref Funder Registry ID: Award Number: 2016-04337Funding provided by: Australian Research CouncilCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100000923Award Number: SGS28/99Funding provided by: Australian Research CouncilCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100000923Award Number: DP130104314Funding provided by: Swedish Research CouncilCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100004359Award Number: 2016-03912Funding provided by: FormasCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100001862Award Number: 2018-01621Funding provided by: Swedish Research CouncilCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100004359Award Number: 2018-05973Funding provided by: NERCCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100000270Award Number: NE/P01903X/1

Includes: TABLE S1: Physiognomic information of modern floral calibration sites and Australian Eocene fossil sites TABLE S2: Modern calibration CCA scores; vector scores and climate; used to calculate regression equations to calculate paleoclimate of fossil sites. TABLE S3: Modern calibration sites in northern Australia for LAA Eqn 4 TABLE S4: Average of natural log of leaf sizes at each locality used in Leaf Area Analysis TABLE S5: Leaf margin percentage and number of leaf types per site; used in Leaf Margin Analysis TABLE S6: Taxa recorded at fossil site; their modern climatic niche and raw distribution data link (GBIF DOI) TABLE S7: Raw climatic output from point extractions of DeepMIP Global Circulation Models at different CO2 levels